1. Field of the Invention
The present invention relates to a sodium-sulfur cell, particularly a sodium sulfur cell having a solid electrolyte tube joined to an insulative ring which electrically insulates a positive electrode chamber from a negative electrode chamber, and a method of joining the solid electrolyte tube and the insulative ring.
2. Related Art Statement
A sodium-sulfur cell is a sealed type high temperature secondary cell which is operated at a high temperature of 300.degree.-350.degree. C. having a sodium ion-conductive solid electrolyte tube made of .beta.-alumina, .beta."-alumina or the like for separating sodium which is an active substance of the negative electrode from sulfur which is an active substance of the positive electrode.
A structure of a conventional sodium-sulfur cell is shown in the attached FIG. 4a, having a solid electrolyte tube 1, a negative electrode chamber 6 filled with sodium arranged at the inside of the solid electrolyte tube 1, a positive electrode chamber 5 filled with sulfur arranged at the outside of the solid electrolyte tube 1, and an insulative ring 2 made of .alpha.-alumina joined to the upper end of the outer circumferential surface of the solid electrolyte tube 1 by means of a solder glass 9. Reference numerals 3 and 4 are metallic lids made of aluminum or coated with aluminum joined under pressure and heating to the bottom surface and the top surface of the insulative ring 2 for covering the positive electrode chamber and the negative electrode chamber, respectively. Reference numeral numeral 7 is a cell vessel, and 8 is a negative electrode terminal tube.
A joining portion of the solid electrolyte tube 1 and the insulative ring 2 of FIG. 4a is shown in enlarged view in the attached FIG. 4b, wherein reference numeral 9 is a solder glass made of an alumina-borosilicate series glass, etc.
Though the sodium-sulfur cell of conventional structure of FIGS. 4a and 4b has the positive electrode chamber 5 and the negative electrode chamber 6 at the outside and the inside, respectively, of the solid electrolyte tube 1, the sodium-sulfur cell may also be formed by arranging the negative electrode chamber 5 and the positive electrode 6 vice versa, namely at the inside and the outside, respectively, of the solid electrolyte tube 1.
Because the sodium-sulfur cell is a type of cell which is operated at a high temperature of 300.degree.-350.degree. C., it undergoes a large thermal change due to start and stop of its operation, and the amount of sodium ions which is the active substance is increased or decreased by transfer thereof through the solid electrolyte tube 1 due to charge and discharge of the cell. Consequently, the joining portion of the solid electrolyte 1 and the insulating ring 2 is liable to suffer from thermal or mechanical stresses, and cracks and damage are likely to occur at the solder glass 9 or the solid electrolyte tube 1.
Particularly, the joining portion of the solid electrolyte tube 1 and the insulative ring 2 of the conventional sodium-sulfur cell as shown in FIGS. 4a and 4b is more liable to suffer from to thermal mechanical stresses at a joining portion of the solder glass 9 and the upper end corner la of the solid electrolyte 1 due to an internal stress of the solder glass 9 when the solder glass 9 is corroded by metallic sodium or sodium vapor, and a danger of inducing a crack 16 in the solder glass 9 as shown in FIG. 4b is increased. Thus, a very dangerous state is likely to occur that sodium or a sodium vapor invades from the negative electrode chamber to the positive electrode chamber 5 through the crack 16 to directly react with sulfur or a sulfide which is the active substance of the positive electrode thereby to generate an extraordinary excessive heat which further broadens the crack or breakage 10.
Moreover, the solid electrolyte tube of a bottomed tube shape separating the positive electrode chamber from the negative electrode chamber is joined at its upper end to the upper end of the positive electrode chamber via the insulative ring made of .alpha.-alumina for electrically insulating the positive electrode from the negative electrode, and the joining portion is exerted by various stresses at the time of ascent or descent of the cell temperature, so that a superior mechanical strength is required to achieve a joining portion which is not broken for a long period of use. However, the makers of joining portion of the conventional cell have not paid attention to such consideration, so that the joining portion is rather weak in mechanical strength and poor in corrosion resistant property. Accordingly, cracks are generated which allow direct reaction of the active substances of the electrodes with each other resulting in problems of overheating of the cells.
As a solder glass 9 at the joining portion used for joining the solid electrolyte tube 1 and the insulative ring 2, an alumina borosilicate series glass is usually used. The joining is effected by a method of inserting the lower portion of the solid electrolyte tube 1 through the insulative ring 2, inserting a glass ring into a gap formed between the solid electrolyte tube 1 and the insulative ring 2, and then heating and melting the glass ring in an electric furnace. However, joined bodies of the solid electrolyte tubes 1 and the insulative rings 2 have a large fluctuation in mechanical strength and are deficient in reliability.